As the year 1974 progresses, a young Jeffrey Dahmer finds himself living in Bath, Ohio, enjoying the comforts that come with family life, which includes his parents, Lionel Dahmer and Joyce Dahmer, as well as his younger brother Dave. Jeffrey's life is marked by an overpowering curiosity that morphs into a disturbing obsession with a jogger he often spots from the window of his school bus. In his spare time, he embarks on an unsettling hobby of collecting deceased animals, which he meticulously dissects using chemicals procured from his father, owing to his fascination with the complex structures of animal anatomy.
By 1978, Lionel's patience is stretched thin with Jeffrey's peculiar bone collection, leading him to issue an ultimatum: Jeffrey must make friends at school. As he attempts to navigate this social environment, he inadvertently attracts attention for mimicking the peculiar mannerisms of his mother’s interior designer, who has cerebral palsy. This odd behavior piques the interest of aspiring artist John "Derf" Backderf and his group of friends. They begin to engage in a series of comedic misadventures, which would later be adapted into Derf's graphic novel, My Friend Dahmer. They form the "Dahmer Fan Club," exploiting Jeffrey’s innocence through pranks that they amusingly refer to as "Doing a Dahmer." Their antics involve sneaking him into every yearbook photo and persuading school authorities to allow them to meet Vice President Walter Mondale during a field trip to Washington, D.C.
As Joyce's mental health deteriorates, the tension between her and Lionel escalates, leading to bitter domestic disputes. To cope with the escalating family issues, Jeffrey turns to heavy drinking and begins venting his frustrations on defenseless animals. The jogger who has captured Jeffrey's fascination is revealed to be Dr. Matthews, a physician treating one of Derf’s friends. In a desperate bid for connection, Jeffrey fakes an illness to procure an appointment with Dr. Matthews, anticipating a glimpse of his unclothed body. During their interaction, Dr. Matthews becomes perturbed by Jeffrey's evident arousal. In the solitude of his home, Jeffrey entertains morbid fantasies involving Dr. Matthews and begins an unsettling stalking campaign against the physician, wielding a baseball bat, yet ultimately does not act on his dark impulses.
As Jeffrey's family life continues to unravel, his father departs the household, leaving Jeffrey to feel isolated and disconnected from his friends. At his graduation, Lionel hands Jeffrey the keys to the family Volkswagen Beetle, which foreshadows its grim future as an instrument of his first murder. While Joyce ultimately leaves Ohio for Wisconsin, abandoning both Dave and Jeffrey, the latter is plunged into a profound loneliness.
Under the cloak of night, Derf becomes privy to a haunting scene: Jeffrey emerges from the shadows, his fingers stained with ominous streaks of crimson. Seizing the chance to engage with this mysterious figure, Derf offers him a ride home, only to uncover a desolate existence lacking purpose. As Derf prepares to leave for college, he shares his sketches with Jeffrey, but the gesture is met with indifference. As the night unfolds, Jeffrey extends an invitation for a beer, which Derf declines, feeling an unsettling aura around Jeffrey. As he retreats to his car, his gaze lingers on the baseball bat ominously positioned on the porch, a stark reminder of the growing chasm that separates him from Jeffrey.
Fast forward two weeks, and Jeffrey's life collides with that of Steven Hicks, an unsuspecting hitchhiker whose unknowing choice seals his doom. As the credits roll, viewers are left with a chilling epilogue: Hicks vanished without a trace, foreshadowing the horrors that would ultimately unfold when Jeffrey Dahmer confronted justice in 1991, confessing to the grisly murders of 17 men.
